In the field of electrophotography it is a well known art to employ a corona producing electrode in proximate relationship with a chargeable electrophotographic material. For example, when an electrochargeable surface is prepared on a face of a foil-like substrate, the usual practice is to induce a corona discharge between the electrode and the substrate surface by arranging the corona electrode with an electrode-to-surface spacing around one centimeter. A high direct current voltage level between three and perhaps nine kilovolts is applied between the electrode and the surface, usually with the corona electrode connected to the most negative terminal. Thereby a corona discharge is established, as it is commonly expressed, between the corona electrode and the surface. The noticeable effect is the ionization of the atmosphere, usually air, with the attendant ozone production and slight trace of purplish-blue light glow.
The prior art teaches methods to stabilize this discharge for better control of the substrate charge values. In particular, the instant inventor, as well as others, have employed the constant current regulation of the corona electrical discharge value, adjustment of voltage potential, and other such means which serve to act upon the corona so as to compensate for changes in the corona value.
The practicing artisan's search for improvement over known methods will be immediately arrested by the novelty offered by this invention. The modulation of the charge laid down on the electrophotographic surface is accomplished by way of altering the speed or rate by which the chargeable surface is caused to traverse past the corona field. This is to say: if the corona field is weak, the translational relationship between the two elements will be slow, whilst should the corona field increase then in a like way the translational rate will increase. The result is the effective energy per unit area on the electrochargeable surface will remain about the same irrespective of the absolute corona current value.
The practice of this invention includes a novel corona-responsive probe located adjacent to the corona electrode and effective therewith so as to receive some small ancillary sample of the true corona current without negating the uniformity of the main corona charge reaching the electrochargeable surface. This sampled current then serves to guide a control loop which operates so as to increase the effective speed of a drive motor in proportion to any increase in corona current, or vice-versa.
The merit in this new approach becomes apparent when the artisan considers that parameters which are for the most part uncontrollable tend to strongly influence the true corona current or charging field reaching out from the corona electrode to the electrochargeable surface. In particular, atmospheric pressure and humidity have a strong effect on the corona field. The earlier art methods of constant current control or voltage adjustment ofttimes elude the attainment of good compensation for these corona affecting influences, thereby comforting an operator into the belief that the end result: that of achieving a finite unit area charge on the substrate is being accomplished when, in reality, it is not, due to sneak current paths which rob much corona current in common cost-effective physical arrangements used for supporting the corona electrode structure. Such paths serve to shunt the corona current and the nature of such parasitic current paths is that they are usually non-linear with corona potential and time, i.e. dirt, for example, on insulators or the like varies in amount and composition with time.